The present invention generally relates to avionics electronics. More specifically, the present invention relates to methods and devices for qualifying Mode-S signals and for controlling reply by transponders to Mode-S interrogations.
A variety of transponders exist today for use with the Air Traffic Control Radar Beacon System (ATCRBS) and that support various communications protocols, such as Mode-C, Mode-A and Mode-S communication's protocols. ATCRBS ground stations, each have a rotating radar dish that transmits a beam having directional characteristics to transmit and receive information to and from aircraft within the relevant air space. The radar dish transmits over a common frequency to all of the aircraft. Each ATCRBS ground station also includes an omnidirectional antenna co-located with the directional radar dish. The omnidirectional antenna transmits, over a control frequency, among other things, side lobe suppression (SLS) signals which, as explained below in more detail, are synchronized to, and used in combination with, transmissions over the common frequency from the directional radar dish. The SLS signals are utilized to prevent aircraft outside of the beam from replying to transmissions generated by the directional radar dish. The aircraft transponders compare certain pulses transmitted from the radar dish over the common frequency with certain pulses transmitted from the omnidirectional antenna over the control frequency. The aircraft transponder determines whether to reply to received signals depending upon the relation between the compared pulses.
In general, an ATCRBS ground station sends approximately 250 to 450 Mode-S interrogations per second per radar frequency. In a ten second period, the radar dish will maintain a specific aircraft within its radar beam for no more than approximately 100 milliseconds which enables approximately 25 to 45 replies to be received by the ATCRBS ground station from each aircraft during each sweep of the radar dish.
To partially address the clutter of the air communication space created by excessive and unsolicited replies, the Mode-Select (Mode-S or discrete beacon address system, DBAS) was developed which permits active transmission of information to and from the aircraft. Mode-S transmissions have greatly reduced the transmission interference or garble previously experienced. In a Mode-S system, the ground station transmitter/receiver interrogates aircraft discretely based on specific 24 bit address assigned to each aircraft. The ground station transmits a Mode-S signal to each aircraft from which a reply is sought. The Mode-S protocol was developed to operate within the existing Mode-A or Mode-C environment.
The ground station produces a tag for each aircraft in its surveillance area through the use of two different methods in order to individually address each aircraft. In one method, a Mode-S SQUITTER is transmitted by the aircraft transponder pseudo randomly with a unique identification code for the aircraft embedded in the transmission. In the other method, a Mode-S ALL CALL signal is transmitted by the ground station. When the ground station transmits an ALL CALL signal, the Mode-S signal includes an interrogation command intended to elicit a reply from the transponders of every aircraft that receives the interrogation command. Each transponder that receives the ALL CALL signal replies by transmitting its unique 24 bit address.
The protocol for the Mode-S signals includes an identifying preamble containing two pulses, namely a P1 pulse and a P2 pulse, separated by a predetermined time interval. The P1 and P2 pulses are transmitted in accordance with a particular pulse width, modulation technique, and frequency. When transmitting a Mode-S interrogation, the ground station after transmitting the P1 and P2 preamble pulses, transmits a differential phase shift keyed (DPSK) data segment of predefined length, such as 56 or 112 bits or chips. The DPSK data segment contains, among other things, the interrogation command. The DPSK data segment includes 24 parity bits to provide a cyclic redundancy check (CRC). The DPSK data segment and CRC bits are embedded within a P6 pulse. The P6 pulse also contains a synchronization phase reversal (SPR) signal that precedes the first data bit/chip by a predetermined time set forth in the protocol.
The aircraft and ground station operate asynchronously with respect to one another since the aircraft transponder is driven by its own internal clock that operates independent of the clock used to drive the ground station transmitter/receiver. This is why, the aircraft transponder first synchronizes incoming received signals with the clock of the aircraft transponder before being able to read the DPSK data segment contained within the P6 pulse of the Mode-S signals. Signals received at the aircraft transponder represent a collection of signals transmitted from different sources, for different purposes and in varied formats. The aircraft transponder searches the collective incoming signals for various identifiers, such as Mode-A, Mode-C and Mode-S indicators. A Mode-S signal is identified by its preamble and more particularly by the pulse width and interval between P1 and P2 pulses. When the transponder detects a valid Mode-S preamble, the transponder next searches for the P6 pulse containing the DPSK data segment. To demodulate the DPSK data segment, the transponder must first be synchronized with the phase of the received Mode-S signal. The transponder achieves synchronization by first identifying the SPR signal contained within the P6 pulse. At the ground station, the P6 pulse is formatted such that the DPSK data segment is transmitted by a predefined time interval after the SPR signal.
At the aircraft, the transponder continuously monitors received signals and, upon receipt of valid P1 and P2 pulses, begins searching the received signal for a P6 data segment and once located, begins searching for the SPR signal. The transponder must detect the SPR signal within an allotted time window following the leading edge of the P6 pulse. Once the aircraft transponder identifies an incoming Mode-S preamble and locates the subsequent corresponding P6 signal and the SPR signal, the transponder is able to become synchronized with the DPSK data segment. If the SPR signal is not received within the allotted time window, the transponder determines that the received signal is not a Mode-S signal.
However, existing transponders have met with certain limitations. As noted above, when a ground station transmits a Mode-S ALL CALL interrogation, it is desirable for an aircraft to reply only when the aircraft is within the radar dish beam. It is preferable that aircraft outside of the radar dish beam not reply as such communications unduly garble the transmission airspace and are not properly receivable by the radar dish. In an attempt to limit aircraft replies only to aircraft within the radar dish beam, a protocol has been defined that must be satisfied by received signals at the aircraft transponder before replying. At the ground station, the radar dish transmits the P1, P2 and P6 pulses over the common frequency for a Mode S interrogation. The omnidirectional antenna also transmits a P5 pulse over the control frequency (as explained below in more detail). The P5 pulse is transmitted in all directions uniformly by the omnidirectional antenna. Thus, the strength or amplitude of P5 pulse received by a particular aircraft is independent of the angular relation between the omnidirectional antenna and the aircraft. In contrast, the P1, P2 and P6 pulses transmitted by the radar dish are directional and thus, signal strength is stronger within the beam formed by the directional radar dish. Hence, the strength or amplitude of P1 and P6 pulses received by an aircraft is dependent upon whether the radar dish beam is directed at the aircraft or not and where the aircraft is located within the beam (e.g., the center or edge).
While the P6 signal is strongest within the beam of the radar dish, aircraft located outside of the beam may still detect the P6 pulse, albeit at a lower signal strength. Consequently, when the ground station transmits an ALL CALL interrogation intended only for aircraft within the beam of the radar dish, aircraft outside of the beam may detect this P6 pulse. In an attempt to prevent replies from aircraft located outside of the beam of the radar dish, the omnidirectional antenna transmits the P5 pulse at a point in time and with a signal shape based upon the timing and shape of the SPR signal in the P6 pulse. The P5 pulse is transmitted to overlap the transmission by the radar dish of the SPR signal. The P5 pulse begins slightly before and continues slightly beyond the SPR signal. By timing and formatting the P5 pulse in this manner, the ground station attempts to overlap the P5 pulse with the SPR signal of the P6 pulse to achieve side lobe suppression (SLS) during Mode-S ALL CALL interrogations. Side lobe suppression is intended to prevent aircraft located outside the beam of the radar dish from responding to the Mode-S ALL CALL interrogation.
As noted above, the amplitudes of the P5 and P6 signals are different, such that the P5 pulse is much smaller than the P6 pulse for aircraft within the radar dish beam. In contrast, aircraft outside of the radar dish beam detect a much weaker P6 signal (due to its directivity). As P5 pulses are transmitted with equal strength in all directions, the relative strengths of P5 and P6 pulses detected by aircraft outside of the radar dish beam are much closer. In fact, the P6 pulse becomes so weak for aircraft outside of the radar dish beam that the P5 pulse becomes greater in strength. As the strength of a received P6 pulse decreases, the P5 pulse amplitude approaches and exceed the amplitude of the SPR signal. Since the P5 signal is much stronger than the SPR signal no phase transitions will be detected within the allocated SPR acceptance window, or thereby obliterating the SPR signal received by an aircraft located outside of the radar dish beam. In such cases, the aircraft is unable to detect an SPR signal within the collective received signal.
However, conventional transponders have experienced difficulty in correctly processing the P5 pulses and SPR signals in a manner that achieves proper cancellation. More specifically, the transponder develops the SPR timing based on the leading edge of the P6 or P2 signals which is detected through pulse amplitude demodulation. The SPR signal and therefore the P5 pulse is detected through a phase demodulator. These two demodulation methods and circuits are sampled through a series of logic circuits which cause the timing of the SPR window to be asynchronous. Consequently, the received P5 pulse (demodulated with DPSK demodulation) is asynchronous to the SPR acceptance window (based on amplitude demodulation) and therefore the P5 pulse jitters with respect to the SPR acceptance window. This problem is further caused by the phase changes in the rising edge of the P5 pulse. These phase changes will also cause the P5 pulse to jitter with respect to the SPR acceptance window. As the P5 pulse jitters with respect to the SPR acceptance window within the circuits of the transponder, this reduces the ability of the P5 pulse to cancel out the SPR signal. This timing error has caused conventional transponders to incorrectly identify SPR signals as valid, where such SPR signals should have been cancelled by the P5 pulse. Incorrect identification causes transponders to reply to ALL CALL interrogations not intended for the particular aircraft.
The tendency for a transponder to reply to unintended ALL CALL interrogations varies based upon the relative strengths of the received P5 and P6 pulses. This tendency has been recognized and led the establishment of standards to which transponders must comply. For instance, when the P6 pulse is at least 12 dB (decibels) greater than the P5 pulse in the received signal, the aircraft is more than likely within the radar dish beam and thus the transponder must reply. If the P6 pulse is 6 dB or greater than the P5 pulse (but less than 12 dB greater than the P5 pulse), then the aircraft is potentially along the edge of the radar dish beam and thus the transponder is required to reply 95% of the time. When the P6 pulse is at least 3 dB greater than the P5 pulse, (but less than 6 dB greater than the P5 pulse), the aircraft is more than likely outside of the beam of the radar dish and the transponder is expected to only reply 50% of the time. When the P5 pulse is 3 dB or greater than the P6 pulse, the aircraft is almost certainly outside of the beam of the radar dish and thus the transponder must reply less than 1% of the time. These broad standards are used as benchmarks for qualifying transponders.
Conventional transponders experiencing the internal timing problems discussed above have difficulty in satisfying these reply standards. A need remains for an improved transponder able to more accurately discriminate SPR signals from noise, to avoid phase shifts between the SPR signal and P5 pulse in the received signal, and to better satisfy the reply standards provided by the ATCRBS system. It is an object of certain embodiments of the present invention to meet one or more of these needs and to meet other needs that will become apparent from the present application.